Tuesday, June 12, 2012

Rain = Silt

Two years ago next month I wrote a post that touched on the silt problems threatening the lifespans of Taiwan's dams. The torrential rains we're getting now, plus the massive downpours we can expect from the typhoon on its way in time for the weekend, suggests that our reservoirs will take another blow..... the article that inspired that post noted...

Before Shihmen Reservoir was completed, engineers estimated that silt would flow into it at a rate of 790,000 cubic meters per year, giving it a useful life of at least 71 years. The reservoir began filling with water in May 1963, but when Typhoon Gloria struck northwestern Taiwan in September of the same year, it washed more than 19 million cubic meters of silt into the new reservoir, equivalent to one-third of its silt capacity. This knocked 23 years off its life expectancy at a stroke.

In August 2009, Typhoon Morakot struck Taiwan with record-breaking rainstorms and devastating floods. As a result, the Cengwun and Nanhua Dam were ravaged, the massive flood water caused Cengwun Dam to silt up with 90 million cubic meters of mud, while Nanhua also suffered silting up of 17 million cubic meters of mud. Consequently, the water retaining capacities of the dams were significantly reduced, seriously affecting the water supply in the Greater Tainan region.

Following Typhoon Morakot last August, the bed of the Laonong River (荖濃溪) in southern Taiwan rose in some places by more than 23m, and the cross-watershed transfer project built to supply water to the Zengwen Reservoir (曾文水庫) in Taipei County was almost completely ruined. That was followed on March 4 by the Jiasian earthquake, after which environmentalist groups found that the transfer conduit tunnel was crossed by four geological faults, presenting the danger of collapse in case of a powerful earthquake.

The conduit is discussed in detail in this 2009 post on The View on the destruction of Hsiaolin Village. The DPP legislator's piece stated that Tsengwen Reservoir's current capacity is less than 40%; it is so shallow that in the rainy season it often overflows and has to be lowered. UPDATE: Mike Fagan, who has traveled all over the south looking at its dams, reminds me that the 2010 program to combat the silting problem of the Tsengwen and Nanhua Reservoirs was implemented and $16 billion was spent on it. A 2011 piece from the China Post notes an important connection between the reservoirs and the construction-industrial state:

The Ministry of Economic Affairs (MOEA) had earlier planned to remove 52 million cubic meters of sand and mud from December 2010 to November 2011 but is now shortening the plan's completion time to the end of June.

....

According to Wu, the government's dredging efforts have achieved good results, with sand prices having fallen by almost 25 percent from July to December last year, lowering the costs of public construction projects.

Heh. Fagan told me of the tributary widening: "you can see the difference - the newer weirs are much wider than the older stuff, and use different designs." Here are his posts on his trips to Nanhua and Tsengwen.

To exacerbate things even further, extreme weather patterns have become ever more prevalent because of global warming. Both the Research Center for Environmental Changes at Academia Sinica and the United Nations-affiliated Intergovernmental Panel on Climate Change have already confirmed that every time the world’s average temperature rises by one degree Celsius, the amount of moisture in the atmosphere increases by 7 percent. This means that convection has intensified and there are now more severe heavy downpours and fewer light showers.

What is more, after analyzing Taiwan’s rainfall data from the last 45 years, Academia Sinica discovered that over this period the number of “severe” and “extremely severe” downpours in Taiwan has increased by 100 percent. The island has thus been affected by extreme weather conditions to a far greater degree than the global average.

If four typhoons strike Taiwan every year, and each typhoon affects the nation for two days, then 40 percent of annual precipitation would fall in eight short days. The number of days when it rains more than 1,000 millimeters per day has also become more frequent. But most of this rainfall flows rapidly directly to the ocean, and very little remains. All this means that the situation whereby Taiwan “rains a lot, but has very little water to use” is getting worse.

On the other hand, the number of days when there has been a light drizzle or sprinkle has declined sharply. Statistics show that during the last few decades, moderate showers fell an average of 70 days per year. In the last few years, this number has gone down to fewer than 30 days per year. Light rains moisten the earth, gradually percolate underground, then slowly migrate to rivers, and finally fill dams with water. This regulating system has today been weakened and is coming to a halt, so that the number of days when rivers run dry has increased, and droughts are becoming more common. The land is undergoing desertification, the rivers are filled with mud and dams are becoming silted.

The bashing by severe rains washes more silt down, meaning that in our warming world dams will have shorter life spans and serve up less water.
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3 comments:

I think there's good reason to be more cautious than your conclusion that...

"The bashing by severe rains washes more silt down, meaning that in our warming world dams will have shorter life spans and serve up less water."

... for several reasons.

Firstly, credit where credit is due: the authorities did carry out a major de-siltation program at both Tseng-wen and Nanhua reservoirs post Morakot, and although not all of the ecologically excess sediment was removed, an awful lot was removed (I think I remember finding the numbers, but I can't remember what they were now) and both reservoirs seem to be functioning reasonably well, if not quite at 100%. In addition, as you mention, the authorities also took preventative measures in addressing the tributaries from where the excess sediment is washed down. I have pictures of the old vs new weirs if anyone wants to see. So... the State authorities can mitigate the over-sedimentation threat to a considerable degree when they put their minds to it and this alone weakens your conclusion.

Second, there are other things that can be done (and in some cases have already been done) about this. If the tributary widening proves insufficient in preventing further over-sedimentation of the Tseng-wen and Nanhua reservoirs, then one thing that could be tried fairly easily and for very little money would be to line the back of the weirs with bundles of rice-straw which would then absorb the finer silt particles from that quantity of the water passing through rather than over the weirs. The authorities have presumably already thought of this, but I'm not sure they have bothered with it as if they had, I'd have expected to find nearby huts-and-trucks to store the stuff. More generally, filtration systems will be the way to go and are a major engineering challenge for the future. But they would not be unprecedented. For other, geographical and engineering reasons, Renyitan reservoir in Chiayi already has a filtration system between it and its source - the Bazhang river. It is consequently one of the healthiest (95% capacity) and least flood prone reservoirs in Taiwan. It is however small, and its geography makes it atypical. Nonetheless, it may be that the relative success of Renyitan's design will inform the design for the long-awaited "Great Lakes" project in Kaohsiung.

The over-siltation at Shihmen reservoir seems to have been the result of a lack of forethought to anticipate this problem. I think eventually, the WRA will have to bite the bullet and turn the taps off for a while in order to get it sorted out (i.e. de-siltation along with repairs to the dam itself and preventative measures like tributary widening and improved weir construction). If they do turn the taps off for six months or a year, it could nonetheless open up a huge market for advanced micro-filtration systems which allow for vastly improved small-scale water recycling.

So there are reasons to think the excessive rainfall can be coped with - in the south, Renyitan, Tseng-wen, Wushantou and Nanhua reservoirs should all be able to cope now I would think. The ones to worry about may be those reservoirs with gravity-arch dams (behind which the silt can pile up) and either no, or inadequate, filtration systems - Shihmen, Deiji and maybe Feitsui in particular come to mind.

Actually, your typical earth-embankment or rock-fill dam is specially designed to prevent this from happening. Although it is true that the shell immediately behind the outer dressing will often be semi-permeable so that some water will get into the dam, these shells are separated from the central core by a drainage channel shaped in a right angle such that the upstream face of the dam would form the hypotenuse. These drainage channels lead straight back into the reservoir itself and prevent the water from building up inside the dam.

The spillway which accompanies any dam is also critical to prevent the build up of pressure and sheer force from too much water behind the dam. At Nanhua reservoir, the spillway is supplemented by two additional escape chutes - one running to the south of the spillway and exiting directly into the spillway pool, and one running beneath the dam itself to the north at a diagonal, and issuing out into a small channel which winds its way back down toward the river as it leaves the spillway pool. Similar supplementary escape chutes are also in place at Tseng-wen, Renyitan and Mudan reservoirs, though I don't know about the gravity-arch dams up north like Deiji and Feitsui (I forgot in my previous comment - Shihmen is also a rock-fill dam, not a gravity arch dam).

What the author in that quote should have said was that the excess sediment fills up the reservoirs rather than "fill up the dams with water".

Another small thing since I've already tarred myself with two comments here...

I was just checking the hydrological data (June 12th) for Mudan reservoir and the water level is 9.43 meters below maximum, which, if you do the math over its area of approximately 1.4 km2 gives you an as yet unfilled volume of nearly 13.4 million m3. The WRA's data also states that the reservoir is currently holding 15.9 million m3 of water, so (bearing in mind that the max capacity of Mudan is 30 million m3) if we work those numbers we get a possible sedimentation value of 700,000 m3 or just over 2%.

So it's easy to overstate the sedimentation problem on force of presumption alone without actually checking.

Of course, with the current heavy rains likely to continue the rest of the week, things could get worse - although the flooding seems to be mainly up north. Down here in Tainan, the usual flood-prone areas of Yongkang and Rende are dry at the moment (or they were when I drove around to check yesterday and the rain since then has been intermittant).

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